In the case of fingerprint sensors, it is known for a sensorally sensitive surface to be provided, which is approximately the same size as a fingerprint. During use, the finger is placed on the sensor surface, and the fingerprint is then read by the appliance. Most fingerprint sensors in use a capacitive semiconductor sensor which identifies a specific capacitance value depending on the local profile of a finger, and uses this to create image information. Owing to the size of the sensorally sensitive surface, semiconductor sensors such as these are very expensive. There is therefore an aim of producing fingerprint sensors which have a smaller sensorally sensitive surface.
EP 0 813 164 A1 discloses the provision of a strip sensor over which a finger is moved. Image strips of the finger are recorded while the finger is being moved over the sensorally sensitive surface. The image strips are produced in a rapid sequence, so that successive image strips overlap, provided that the speed at which the finger is moved does not exceed a specific limit value. A complete area image of the fingerprint is then produced on the basis of these overlaps. In a subsequent identification stage, the reconstructed fingerprint is compared with a reference fingerprint in an analogous manner to the procedure for images recorded by means of an area sensor. This is therefore a two-stage method, in which an area image is first of all reconstructed from image strips, and conventional image comparison methods are then applied.
An undistorted image of the finger is created using the scanned in image strips and, for example, the speed at which the finger is drawn over the sensor surface. In order to make it possible to implicitly determine the speed at which the finger is drawn over the surface from the image strip sequence, successive image strips must overlap. Individual image strips are in turn obtained row-by-row in a sequential process, with the speed at which the rows are read being referred to as the scanning rate. It can be shown that successive image strips will overlap only when the speed at which the finger is drawn over the surface is less than half the scanning rate. This limit value on the speed at which the finger is drawn over the surface greatly restricts the intuitive nature of the usefulness of the sensor. If the speed is exceeded, then the speed at which the finger is drawn over the surface can no longer be reconstructed, or parts of the area image information may even be missing, so that severe distortion can occur in the image reconstruction, making identification impossible.
A further disadvantage of the known method is that, in principle, the reconstruction is not very robust since successive image strips in a strip sequence are only ever reconstructed in pairs. If there is an error in the overlap between two strips, then this error is propagated over the rest of the entire image. Further errors at later times are also additive. In summary, this means that an error may occur which extends over the entire reconstructed image and endangers the subsequent identification if the image information relating to just one single strip is disturbed or its image quality is poor.
A third disadvantage is that the image reconstruction is computation-intensive. Since the subsequent area image identification stage remains unchanged, the reconstruction time in a system such as this is added to the identification time, resulting from the use of an area sensor.